In order for laser light to be useful, it must exit from the internal resonator cavity wherein it is produced. For low power lasers, the construction of a suitable output window presents no serious problems. A material is selected which is reasonably transmissive at the wavelength of laser operation, and the desired output window is fabricated. Typically, there is no need for special cooling of the window since the incident flux of light energy is small, well within the tolerance of most window materials.
The situation changes drastically for high-power lasers, typically in the range of one kilowatt laser power and up. For such powerful lasers it becomes a matter of concern to remove the laser power from the internal resonator cavity. A laser window made of transparent material is still typically employed. However, any minute imperfection in the material (scratch, micro-crack, dirt, bubble, etc) will preferentially absorb laser energy. At high powers, the laser energy absorbed will significantly heat the local region around this imperfection, further enhancing the preferential heating. The net result is a thermal runaway process, leading to intense local heating, large thermal gradients in the window, and ultimately cracking and catastrophic window failure.
Even optically perfect windows have certain problems. No practical window material will be perfectly transmissive. A small fraction of the incident power will be absorbed by the window and cause heating. For low laser powers, such heating is negligible. However, for high laser powers, even modest light absorption can lead to serious heating. Therefore, windows for high-power lasers are frequently cooled by contact with water-cooled metal mounts along their outer circumference. The need for water cooling and the need for very high optical quality materials make output windows for high-power lasers relatively expensive. Even with high quality optical material and circumferential cooling, such windows are subject to thermal damage and need replacing at fixed intervals, contributing significantly to the cost of high-power laser operation.
At still higher levels of laser power, (typically, around six kilowatts) circumferentially cooled windows become inadequate. In order to keep the incident laser power per unit window area below the damage threshold of the window material, the window and laser beam must both be rather large. Thus, circumferential cooling of the window is inadequate to prevent unacceptable heating of the central portions of the window.
The commonly accepted solution to this dilemma is to remove the window entirely, leaving a gap from the laser cavity to the outside, as used by AVCO Corporation in the AVCO Laser Metalworker. In order to avoid seriously detrimental gas flows through this opening, complex arrangements of differential vacuum pumping are required. Nevertheless, leakage of laser gas through this "aerodynamic window" adds significantly to the cost of operation, while the additional pumping contributes to the complexity and cost of the equipment.
One way to make transparent laser windows useful at higher power levels would be to increase the window cooling, allowing larger heat input to the window without damage. Since high-power lasers frequently produce annular-shaped output beams, the central portion of the output window in such lasers is not used for beam transmission. Thus, a device for cooling the central portion of the window, without interrupting the annular laser beam in any way, should increase the heat input (hence, the laser power) that can be tolerated for a given window. Such a device is the subject of the present invention.